Conventionally, ship radar apparatuses that can display an image (radar image) showing target object(s) around a ship and can change a scale size of the radar image have been known. JP08-105954A discloses such a ship radar apparatus. With a large scale size, although only the radar image for a narrow range close to the ship is displayed, detailed position(s) and shape(s) of echo(s) showing the target object(s) can be grasped. On the other hand, with a small scale size, although the radar image for a wide range can be displayed, it becomes hard to grasp the position(s) and shape(s) of the echo(s) in detail.
Therefore, with the ship radar apparatus with the configuration above, the scale size is required to be changed manually according to a situation. In addition to the ship radar apparatus, the change is similarly required for, for example, a radar apparatus installed on the ground (JP05-080143A) and a vehicle navigation apparatus (JP11-132777A).
Note that JP05-080143A discloses a configuration for displaying display targets by using different marks for each kind thereof (e.g., people, combat vehicles, and combat airplanes). JP11-132777A discloses a configuration for displaying a map and landmarks around a vehicle, and in which detailed information on the landmarks (e.g., if the landmark indicates a store, business hours) is displayed.
The scale size is required to be changed according to the situation in the configurations disclosed in JP08-105954A, JP05-080143A, and JP11-132777A, whereas JP09-229710A discloses a ship navigation apparatus that can change its scale size automatically. In the ship navigation apparatus disclosed in JP09-229710A, a route (shipping route) can be set. The set route and a position of a ship are displayed on a display unit provided in the ship navigation apparatus. Further, when the ship moves away from the set route, the scale size is automatically reduced. In this manner, the ship and the route can be displayed at all times without changing the scale size manually.
However, with the configuration in JP09-229710A, although the manual change of scale size is not required when the ship has moved away from the route, in a case of, for example, referring a detailed nautical chart for an area close to the ship, the scale size is required to be enlarged manually. Moreover, in the state with the enlarged scale size, problems may arise that the set route may not be referred and, for example, a port far from the ship may not be referred. Therefore, even with the configuration in JP09-229710A, there is a case where the scale size needs to be changed frequently.